The literatures are cited in the present description for the purpose of describing the state of the art to which the present invention pertains, and are incorporated herein by reference in their entire disclosures.
In 2009, U.S. Energy Secretary Steven Chu proposed that, in order to solve the problem of global warming, all of the nations should paint the building's roofs white as much as possible to increase solar reflectivity of roofs, reduce the greenhouse effect, and achieve the energy saving and decreased consumption.
A construction law is issued in the state of California, US to require the low slope roofing achieves the reflectivity of at least 70%. It is a very efficient heat shield technology to apply sand having high reflectivity onto the surface of the asphalt roofing. Compared with the reflective material such as plastics, metal, and organic coating, sand having high reflectivity has the characteristic of low cost and weather resistance. But, for most of white granules available in the market, such as quartz, calcite, calcined kaolin, and synthetic ceramic granules, the accumulated granules are usually measured to have high reflectivity, but after being applied to a black substrate, due to its low opacity to UV radiation, as well as being usually accompanied by a remarkable oil-absorbing phenomenon, it results in color change, and a further reduction in the reflectivity. In order to achieve the requirement of the reflectivity of 70%, a reflective coating can be applied on the granule roofing product. However, the polymeric organic coating has a short life span and therefore it is necessary to re-apply the coating after several years, such repeated applying results in high cost.
U.S. Pat. No. 8,865,303 B2 discloses a cool roofing system, which includes highly reflective calcined kaolin granules having a reflectance of 80% to 92%. When coated with a polymeric organic coating and applied to a roofing substrate, the highly reflective kaolin granules produce a roofing system having a reflectance of 70% or more.
JPH08-091892 points out that, when the conversion from the crystalline phase of kaolin to crystalline phase of mullite is enhanced, the laminated structure of kaolin crystalline will be broken and the whiteness will be lowered. In order to solve the problem, a part of residual kaolin crystalline can maintain laminated structure by adjusting the grinded particle size, pelletizing or shaping, adjusting the calcination temperature (1500-1700° C.) and the like. This can not only increase the reflectivity at the particle interface and the whiteness, but also increase the opacity and improve the overall reflectivity. However, the above method requires complicated processes such as grinding, pelletizing and shaping, and the calcination should be conducted at a high temperature of 1200-1700° C. Even if the above method is industrialized, it will have the problem of high cost, etc.
Therefore, there is a continuous demand for granules having high reflectivity, low transparency and low oil-absorption.